Beams of ultrashort laser pulses are increasingly used for cutting and drilling a wide range of materials. Traditional mechanical processing produces rough surfaces and unwanted defects, such as micro cracks, which degrade and weaken the processed material. Laser material processing using a focused beam of ultrashort laser pulses produces more precise cuts and holes, having higher-quality edges and walls, while minimizing the formation of unwanted defects. High-energy pulses enable parallel processing, whereby a beam of ultrashort pulses is split into a plurality of beams that are directed to different work stations. High-energy ultrashort laser pulses are also used for high harmonic generation, creating laser pulses having attosecond durations and soft x-ray wavelengths to probe dynamic processes of molecules.
Pulsed MOPA systems are arranged to generate output pulses having a greater pulse energy than is possible with any present form of laser-oscillator. A MOPA typically includes a master oscillator arranged to deliver seed pulses. Seed pulses from the master oscillator are delivered to a power amplifier, which increases the energy thereof to a desired energy, with a higher average power.
A mode-locked master-oscillator typically generates seed-pulses at a pulse-repetition frequency (PRF) that is too high and a pulse-duration that is too short for subsequent amplification. Accordingly, the pulses to be amplified are selected from the pulses generated by the master oscillator by a device commonly referred to by practitioners of the art as a pulse-picker, which typically includes an acousto-optic modulator (AOM). The duration of pulses to be amplified is temporally extended by some wavelength-dispersive device, typically referred to as a pulse-stretcher.
The amplified pulses may be temporally shortened to a desired duration by a wavelength-dispersive device commonly referred to by practitioners of the art as a pulse-compressor. This requires that whatever gain-medium is used in the power amplifier has a gain-bandwidth sufficient to allow effective pulse-compression. Because of high average power of the amplified pulses and a correspondingly high pump-power for the power amplifier, it is also required to provide some form of cooling for the amplifier gain-media.
In power amplifiers having a gain-element made of a solid-state gain-medium, the above discussed requirements create a potential conflict, inasmuch as solid-state gain-media having a relatively high thermal-conductivity, for example greater than about 6 Watts per meter Kelvin (W/(m K)), also have a relatively narrow gain-bandwidth at a peak-gain (peak emission) wavelength, for example less than about 10 nanometers (nm). Examples of such gain-media include yttrium aluminum garnet (YAG), lutetium aluminum garnet (LuAG), and gadolinium gallium garnet (GGG).
Existing MOPAs having an ytterbium-doped YAG (Yb:YAG) gain-medium, operate at the peak-gain wavelength of 1030 nm and provide pulse durations between 500 femtoseconds (fs) and 1 picosecond (ps), at an average power of up to about 100 Watts (W). A regenerative amplifier configuration having an optical switch captures a pulse in an optical resonator, the pulse making a plurality of round trips in the resonator to achieve sufficient amplification, before being released by the same or another optical switch. Regenerative amplifiers are complex to control and are relatively expensive. Each optical switch usually has an electro-optic modulator and at least one polarizer. Pulses having a duration of about 300 fs have been reported, but spectral shaping of pulses is required during each of the round trips through the regenerative amplifier. Typically, more than fifty and sometimes many hundreds of round trips are required to extract the available gain and thereby achieve sufficient amplification. Such spectral shaping creates a loss in the regenerative amplifier that effectively limits the maximum accessible output pulse energy and average power.
Current MOPA architectures based on bulk ytterbium-doped regenerative amplifiers, ytterbium-doped photonic crystal amplifier fibers, or ytterbium-doped rod amplifier fibers do not allow economic scaling of output average power to greater than 100 W for compressed pulse durations below 400 fs. This limits industrial applications for MOPA laser-systems. Accordingly, it would be advantageous to have a MOPA system capable of delivering pulses at an average power greater than 100 W with pulse-durations of less than 400 fs, preferably without using a regenerative amplifier as the power amplifier.